Antiworld flashes into view

作者：娄仪凡 发布时间：2019-02-28 04:19:03

By Peter Aldhous AT the last count, the periodic table contained 111 chemical elements. Its shadowy antimatter counterpart contains just one, now that physicists from Germany, Italy and Switzerland have for the first time created antihydrogen, the simplest antiatom. “This is the first step in the antiperiodic system,” says the team’s spokesman, Walter Oerlert of the Institute of Nuclear Physics at the National Research Centre in Jülich, Germany. “It’s really the proof that there is an antiworld.” Although antiatoms are entirely new, physicists have been studying their constituent parts for years. Like conventional matter, antimatter is built from subatomic building blocks. Antiprotons, the negatively charged antimatter shadows of protons, can be made by smashing protons into one another at immense energies. And positrons, the positively charged counterparts of electrons, are emitted naturally during the radioactive decay of some unstable atomic nuclei. Antimatter is annihilated whenever it comes into contact with normal matter, releasing large amounts of energy. Oerlert and his colleagues have now combined single antiprotons and positrons to make antiatoms of antihydrogen. They used a machine called the Low Energy Antiproton Ring (LEAR) at CERN, the European Centre for Particle Physics in Geneva (see “The race to create an antiatom”, New Scientist, 13 May 1995). For a total of some 15 hours in September and October last year, the physicists fired a jet of xenon atoms across LEAR’s antiproton beam. Collisions between antiprotons and protons in the xenon nuclei generated pairs of electrons and positrons. These positrons then combined with further antiprotons in the beam to make antihydrogen. LEAR is shaped like a running track, with straight sections joined by bends. The researchers created antihydrogen in one of the straights, and placed detectors made of silicon at the beginning of the next bend. Because antiatoms have no charge, the antihydrogen was not pulled around the bend by LEAR’s powerful magnetic fields, and instead careered into the detectors at nine-tenths the speed of light. Inside the detectors, the antiatoms were stripped down once more into their component antiprotons and positrons, which left traces in the silicon. Each positron was also quickly annihilated by an electron, giving off two photons of gamma radiation as it disappeared. These photons shot out of the silicon in opposite directions, and were detected in crystals of sodium iodide held around the silicon. In a paper that will appear later this year in the journal Physics Letters B, Oerlert and his colleagues say that they recorded 11 such combinations of particle traces and gamma radiation. The problem is that other events can mimic the signature of antihydrogen. “It’s a very difficult experiment,” says Michael Charlton, an antimatter expert at University College London. Rolf Landua, a CERN physicist who works on another experiment at LEAR, explains the problem. He says collisions between antiprotons and xenon nuclei would also have produced uncharged antiparticles called antineutrons, which would create almost identical signals in the detectors. Antineutrons, however, penetrate farther through silicon than positrons. By examining where the 11 gamma ray signals appeared in their detectors, Oerlert and his colleagues have estimated that all but two are due to antihydrogen. “I am now more or less convinced that they have seen something,” says Landua, who admits that he was sceptical until he saw this more detailed analysis. So now that antimatter has been made, what is it good for? Spacecraft powered by the annihilation of antimatter are still the stuff of science fiction. But if scientists can make antihydrogen that can be stored for long periods ahd studied in detail, it could be used to search for flaws in the standard model of particle physics – the nearest thing we have to a definitive theory of the Universe’s fundamental components. An atom’s electrons can be made to jump from one energy level to another by probing them with a laser. When they do so, they give off light with a characteristic wavelength. Physicists expect that the spectrum of light given off as the single positron orbiting an antihydrogen nucleus jumps between energy levels should be exactly the same as the corresponding hydrogen spectrum. If it is not, they will have to modify some their most fundamental ideas. “We would have to find something beyond the standard model,” says Landua. In principle, the spectrum of an antiatom could be measured using existing high-precision laser spectroscopy equipment. But this is impossible for the antihydrogen created by Oerlert and his colleagues because each antiatom was destroyed within about 30 nanoseconds of its creation. “It’s travelling so swiftly that there’s absolutely no prospect of stopping it without destroying it,” says Charlton. This is why Charlton and other physicists want to create antihydrogen by another method: they intend to slow down a beam of antiprotons by passing it through a series of metal foils. Once halted,